1. Field of the Invention
This invention relates to an apparatus for measuring the relative position of a mask and a stage in an exposure apparatus.
2. Related Background Art
In recent years, semiconductor integrated circuits have enhance their degree of integration more and more and it has become required that the minimum line width of the circuit be formed in submicrons. In accordance therewith, higher resolving power and higher alignment accuracy have become desired for projection exposure apparatuses for the manufacture of semiconductor circuits.
Now, to enhance the resolving power, it is conceivable to make exposure radiation into radiation of shorter wavelength. So, nowadays, an excimer laser emitting a light in the ultraviolet range, represented by KrF laser, has been attracting attention as a radiation source for exposure.
The excimer laser is a pulse laser of high luminance and high output having many oscillation wavelengths in the ultraviolet range. Particularly, KrF laser oscillating at the wavelength of 248 nm is regarded as being most promising as the radiation source for the exposure apparatus in the next generation.
When alignment of a mask and a wafer is to be effected by the use of the excimer laser as the exposure radiation source, particularly, when off-axis type alignment in which the exposure position and the alignment position differ from each other is to be effected, the measurement of the position of the mask poses a problem. That is, in a conventional apparatus using a super-high-pressure mercury lamp as a light source, where the exposure radiation is used as the mask alignment light, it is easy to accomplish the alignment by a photoelectric microscope or a TV image because the g-line (wavelength 435.8 nm) output from the super-high-pressure mercury lamp is a continuously emitted light, and on the other hand, where the excimer laser is used as alignment light, the alignment by a photoelectric microscope or a TV image is very difficult because the excimer laser emits a pulse light.
However, although the excimer laser is a radiation source which emits a pulse light, it is relatively easy to detect the quantity of light thereof or the peak power thereof by the use of a photodiode or the like. Accordingly, even where the excimer laser is used, it is possible to accomplish alignment by a method as described hereinafter. That is, a regulation plate having, for example, a slit-like opening is provided on a stage holding a substrate such as a wafer thereon, and the regulation plate is illuminated from below it by the excimer laser which is exposure radiation, and the image thereof is formed on a pattern surface. Then, by moving the stage, the corresponding window of a mask is scanned by the projected image of the opening of the regulation plate. In this case, when the projected image of the opening coincides with said window, a maximum quantity of light passes, and the quantity of light decreases in conformity with the deviation therebetween. Accordingly, by recording the variation in the quantity of light in the above-described case, the relative position of the regulation plate, namely, the stage, and the mask can be known.
So, recently, there has been conceived an exposure apparatus which, by the utilization of the above-described principle, is designed to transmit the excimer laser to below the regulation plate by the use of an optical fiber, and illuminate the regulation plate from below it.
In the apparatus wherein, as described above, the excimer laser is applied to the regulation plate through the optical fiber, there has been the problem that there occurs interference of speckles or the like peculiar to a single wavelength light (coherent light) such as the excimer laser, which in turn leads to low measurement accuracy.